The present invention relates to a toner. More particularly, it relates to a toner which is suitably used in image forming apparatuses utilizing an electrophotographic process, such as electrostatic copying machines, laser beam printers and the like.
In these image forming apparatuses, a thermal fixing system, which comprises developing a toner on a photoconductor, transferring the toner to copying paper, and passing the paper through a thermal fixing place to obtain a copied image of fixed toner image, is employed in most cases. Particularly, a thermal rolling system is very often employed under the condition that the surface temperature of the thermal roller is 180 to 200.degree. C., considering heat capacity or heat conductivity of the paper, toner and the like, a setting temperature of which is higher than a softening temperature (120 to 150.degree. C.) of the toner.
With the requirements of high speed and low energy of the electrophotographic process, it has recently been required to develop the toner at a lower temperature, e.g. the surface temperature of a thermal roller of not more than 170.degree. C.
Therefore, there has been made a trial of reducing the molecular weight of a binding resin to lower a softening temperature or reducing a glass transition temperature (Tg) to improve the fixing properties of the toner at a low temperature range. However, since the viscosity is also lowered by reducing the molecular weight of the fixing resin, when toner image is melt-fixed between thermal rollers, a part of the toner is transferred to the rollers, and transferred components cause scumming to the copied image, that is, so-called offset occurs. Furthermore, the toner before image formation is aggregated under a high temperature environment, that is, so-called blocking occurs.
For the purpose of inhibiting hot offset and blocking, there has been suggested a toner prepared by blending releasants such as lower molecular weight polypropylene (e.g. polyethylene, polypropylene, etc.) and natural wax. However, since the fixing resin has small affinity to the releasant, a poorly dispersed wax is released from the toner particles. The wax components thereby cause scumming and melt-adhering to a photoconductor drum, that is so-called filming, a dash mark and the like occur. For preventing toner-blocking, a fixing resin having a comparatively higher glass transition temperature, has been considered but such a method can not be employed because the fixing ability is inhibited.